Skip admin and change oidc user not found message more readable

[stonezdj]

fixes #21041
verified in local env
Thank you for contributing to Harbor!

Comprehensive Summary of your change

Issue being fixed

Fixes #(issue)

Please indicate you've done the following:

• Well Written Title and Summary of the PR
• Label the PR as needed. "release-note/ignore-for-release, release-note/new-feature, release-note/update, release-note/enhancement, release-note/community, release-note/breaking-change, release-note/docs, release-note/infra, release-note/deprecation"
• Accepted the DCO. Commits without the DCO will delay acceptance.
• Made sure tests are passing and test coverage is added if needed.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed in website repository.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.

Project coverage is 66.08%. Comparing base (c8c11b4) to head (06d5924).
Report is 299 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pkg/oidc/secret.go	0.00%	3 Missing ⚠️
src/server/middleware/security/oidc_cli.go	0.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main   #21061       +/-   ##
===========================================
+ Coverage   45.36%   66.08%   +20.71%     
===========================================
  Files         244     1049      +805     
  Lines       13333   114651   +101318     
  Branches     2719     2867      +148     
===========================================
+ Hits         6049    75769    +69720     
- Misses       6983    34735    +27752     
- Partials      301     4147     +3846     

Flag	Coverage Δ
unittests	66.08% <0.00%> (+20.71%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/pkg/oidc/secret.go	24.67% <0.00%> (ø)
src/server/middleware/security/oidc_cli.go	63.63% <0.00%> (ø)

... and 1286 files with indirect coverage changes

[ianseyer]

+1, this would be very helpful

[OrlinVasilev]

+1